Fix unit test for verifying enforced valid expiration date.

The enforced expiration date was not actually enabled, as for that
"shareapi_default_expire_date" needs to be explicitly set to "yes".

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

tests/lib/Share20/ManagerTest.php

@@ -889,6 +889,7 @@ class ManagerTest extends \Test\TestCase {
 			->willReturnMap([
 				['core', 'shareapi_enforce_expire_date', 'no', 'yes'],
 				['core', 'shareapi_expire_after_n_days', '7', '3'],
+				['core', 'shareapi_default_expire_date', 'no', 'yes'],
 			]);
 
 		$hookListener = $this->getMockBuilder('Dummy')->setMethods(['listener'])->getMock();